1. Field of the Invention:
The invention relates to an ion sensitive field effect transistor (ISFET), and in particular relates to an a-WO3-gate ISFET fabricated by RF-sputtering for detecting the hydrogen ions in aqueous solution. In addition, this invention relates to a method for making the a-WO3-gate ISFET.
2. Description of the Prior Art:
The ISFET was first disclosed by P. Bergveld in 1970. The device is a product of applied electrochemistry and microelectronics, and has the function of ion selection and the properties of the FET. This ion sensitive device is strictly different from the traditional ion selection electrode. P. Bergveld disclosed a FET, wherein the metal film set in the gate of traditional FET was removed. Moreover, the device was dipped in electrolyte, wherein no reference electrode was present. However, it was found that a reference electrode must be added to determine the relative voltage between the electrolyte and the semiconductor substrate during detecting so that the ISFET can be operated correctly.
Currently, a large amount of researches on ISFETs are underway. One interesting filed of the research is the use of the membrane for detection. When single gate detecting membrane consists of silicon dioxide, the sensitivity and stability of the device are poor. However, when the detecting membrane of the ISFET consists of double dielectric layers, such as Si3N4/SiO2, Al2O3/SiO2, Ta2O5/SiO2, SnO2/SiO2 and a-WO3/SiO2, the properties are superior to the ISFET with a detecting membrane comprising a single SiO2 gate. Regarding a H+-ISFET, it has been noted that a greater range of ions can be detected when a corresponding detecting membrane is covered on the SiO2 or Si3N4 detecting membrane.
Today, the sorts of the FET based on ISFET process used to detect ions and chemicals have reached more than 30. Moreover, the shrinking, integration, and multi-functionization of the ISFET devices have greatly progressed. The advantages of the ISFET can be summarized as follows:
1. Shrinkage size and micro-solution detectable;
2. High input resistance;
3. Low output resistance;
4. Fast reaction time;
5. Low price;
6. MOSFET processes compatible; and
7. Biosensor applicable.
Current ISFET researches can be classified into six categories:
1. Preparation of sensing membrane by CVD, thermal oxidation, E-gun evaporation, thermal evaporation and sputtering;
2. Shrinkage of the device and the reference electrode;
3. Basic theory, such as site binding model;
4. Packing technique, such as packing material, for example Si-rubber and epoxy;
5. Integration of the devices and circuits; and
6. Simulation of the ISFETs.
A number of patents relating to ISFETs have been obtained, as summarized hereinafter. U.S. Pat. No. 4,385,274 discloses a method and device for compensating temperature-dependent characteristics change in ion-sensitive FET transducer, which is characterized by using a differential system consisting of ISFETs and a circuits-readout module. U.S. Pat. No. 4,609,932 discloses a nonplanar ion-sensitive field-effect transistor device, which is characterized by forming a 3-D ISFET device by laser drilling and solid-state diffusion. U.S. Pat. No. 4,657,658 discloses a semiconductor integrated circuit for sensing a physico-chemical property of an ambient and includes a pair of semiconductor devices having a similar geometric and physical structure, one device being sensitive to the property, the other being insensitive to the property, together with a differential amplifier having feedback connection to one of the pair of semiconductor devices. U.S. Pat. No. 4,812,220 discloses an enzyme sensor for determining a concentration of glutamate, comprising an immobilize enzyme acting specifically to a substrate and a transducer for converting the quantitive change of a substance or heat which is produced or consumed during an enzyme reaction to an electrical signal, wherein the enzyme is glutamine synthetase and the transducer is the pH glass electrode or ion-sensitive field-effect transistor (ISFET). The enzyme sensor can be miniaturized and can accurately determine a concentration of glutamate even when it is low. U.S. Pat. No. 4,839,000 discloses buffer compensation in enzyme-modified ion sensitive devices, which is characterized by using enzyme-modified ion sensitive field transistors to control the compensation of the ions in solution. U.S. Pat. No. 5,319,226 discloses a method of fabricating an ion sensitive field effect transistor with a Ta2O5 hydrogen ion sensing membrane, wherein a Ta2O5/Si3N4/SiO2 dielectric layer, used as the sensor-device, is formed over the gate region of the ion sensitive field effect transistor by RF-sputtering. U.S. Pat. No. 5,350,701 discloses a process for producing a surface gate of an integrated electro-chemical sensor, and the integrated chemical sensor thus produced, wherein the surface gate is particularly sensitive to alkaline-earth species, and more particularly, sensitive to the calcium ion. U.S. Pat. No. 5,387,328 discloses a bio-sensor using ion sensitive field effect transistor with platinum, wherein an immobilize enzyme membrane is immobilized on the ion-sensing film to determine the concentration of glucose. Moreover, a Pt reference electrode is introduced to shrink the size of the bio-sensor. U.S. Pat. No. 5,407,854 disclose the ESD protection of ISFET sensors, which is characterized by providing a method for providing electrostatic discharge protection to ion sensitive field effect transistor.
There has been interest in the material of the sensing membrane of ISFET, wherein a-WO3 is a potential for use as the gate of the ISFET. The a-WO3 is a N-type semiconductor compound, and has both wide energy gap (Eg=2.1xcx9c3.8 eV) and high dielectric constant (xcex5=260 xcex50.). Moreover, a-WO3 is redox reversible, electrochromic and photochromic. Because the range of the resistivity of a-WO3 is large (ranging from 10xe2x88x923 to 1011 xcexa9xc2x7cm), the a-WO3 has potential to be a sensor, such as a gas sensor for detecting the CO2, NO2, H2S, and so on.
Currently, a WO3 layer can be formed by E-bean evaporation, DC or AC sputtering, thermal evaporation, vacuum evaporation, and CVD. The composition of the WO3 layer and its properties vary with the selected method and condition during preparing the WO3 layer. Most of the WO3 layers are amorphous, polycrystalline or crystalline. The composition and structure of the WO3 layer will directly affect its resistivity and electrochromic property; thereby the properties of the devices are determined by the composition and structure of the WO3 layer.
In general, the composition of the WO3 layer is hard to control regardless of which method is used in its formation. For example, the composition of the WO3 layer made by vacuum evaporation is hard to control and the surface of the WO3 layer is not uniform.
Some patents about the usage of ISFET have been disclosed using a H+-FET sensing film including SiO2, Si3N4, Al2O3, and Ta2O5. However, a FET used to detect the industrial effluent is seldom mentioned, and especially a FET used to detect the effluent of low pH. Moreover, the gate of the conventional ISFET consists of a single SiO2 layer; thereby the sensitivity and linearity cannot meet the practical requirements.
One feature of the invention discloses an ISFET comprising a H+-sensing material consisting of a-WO3. The present ISFET is very sensitive in solution, and particularly in acidic solution. The sensitivity of the ISFET of the present invention ranges from 50 to 58 mV/pH. In addition, the disclosed ISFET has high linearity. Accordingly, the ISFET of the present invention is suitable applied to detect effluent.
In order to achieve the above-mentioned feature, the present invention discloses a RF-sputtering a-WO3 gate ISFET and the making thereof. This invention is characterized by forming the a-WO3 layer as the sensing membrane of the ISFET by RF-sputtering, wherein the reactant consists of Ar and O2. That is, the gate of the detecting device consists of a-WO3 layer/SiO2 layer. Comparing the sensitivity of the sensing membrane in aqueous solution with various pH values, the results indicate the device made according to this invention is sensitive to the acidic aqueous solution and has good linearity. Accordingly, the detecting device according to this invention is superior to the traditional detecting device containing a gate consisting of a single SiO2 layer.
Other feature and advantages of the invention will be apparent from the following detailed description, and from the claims.